This invention describes the preparation of improved polyimide gas separation articles, such as films, coatings and membranes, as well as gas separation processes that utilize the membranes. Specifically, soluble polyamic acid salt (PAAS) precursors comprised of tertiary and quaternary amines, ammonium cations, sulfonium cations, or phosphonium cations, are prepared and fabricated into membranes that are subsequently imidized and converted into rigid-rod polyimide articles, such as membranes with desirable gas separation properties. Method of enhancing solubility of PAAS polymers in alcohols is disclosed. Also disclosed are improved methods for thermal and chemical imidization of the PAAS polymer precursory articles. Gas separation processes that utilize polyimide membranes made using PAAS precursers are further disclosed.
The use of polymeric membranes for gas separation applications is well documented in the art. The relationship between the polymeric structure and the gas separation properties has been extensively studied, see for example, W. J. Koros, Journal of Membrane Science, Volume 83, pp1, 1993; L. M. Robeson, Journal of Membrane Science, Volume 62, pp165, 1991; L. M. Robeson, Polymer, Volume 35, pp4970, 1994; and B. D. Freeman, Macromolecules, Volume 32, pp375, 1999. It is well documented in the art that stiffening polymeric backbone while simultaneously inhibiting chain packing can lead to improved gas permeability combined with an increase in gas selectivity for certain gas mixtures. Polyimides are examples of such rigid-rod polymers showing desirable gas separation properties, see for example, D. R. B. Walker and W. J. Koros, Journal of Membrane Science, Volume 55, pp99, 1991; S. A. Stern, Journal of Membrane Science, Volume 94, pp1, 1994; K. Matsumoto, P. Xu, Journal of Applied Polymer Science, Volume 47, pp1961, 1993. U.S. Pat. Nos. 4,705,540; 4,717,393; 4,717,394; 5,042,993; and 5,074,891 disclose the preparation of such aromatic polyimide gas separation membranes.
For practical industrial applications, polymeric gas separation membranes are fabricated into an asymmetric or a composite configuration with thin separation layers. The membranes can be further configured into flat sheets or into hollow fibers. Although rigid-rod polyimides have desirable gas separation properties, they are frequently insoluble or can be dissolved only in aggressive organic solvents which makes it difficult to prepare membranes with ultrathin separation layers and can further cause environmental problems. For example, polyimide membranes have been fabricated from chlorophenol solutions as described in U.S. Pat. No. 4,440,643.
U.S. Pat. Nos. 5,618,334; 5,725,633; and 5,744,575 disclose modified polyimides containing sulfonic acid groups that exhibit improved solubility in common organic solvents. U.S. Pat. Nos. 4,440,4643 and 5,141,642 disclose the process of fabricating polyimide gas separation membranes from polyamic acid precursors. However, polyamic acids can undergo dehydration and are sensitive to temperature and moisture variations, which makes the manufacturing of polyamic acid membranes that exhibit reproducible properties most difficult. Furthermore, some polyamic acids are not soluble in mild organic solvents, and all polyamic acids require harsh conditions to complete imidization. For example, temperatures as high as 300xc2x0 C. are generally required to complete imidization of polyamic acids by thermal treatment. The limited availability of solvent systems and high imidization temperatures prohibit the application of polyamic acid precursors as the coating material for the fabrication of composite polyimide membranes when preferred, readily available polymeric substrates, such as polysulfone are used. To maintain the high level of substrate porosity, the thermal imidization temperature must be lower than the glass transition temperature of the substrate polymer. Most of the commercially employed polymeric substrates have glass transition temperatures below 200xc2x0 C., for example, the Tg of polysulfone is about 190xc2x0 C. G. A. Polotskaya et al. disclosed a polyamic acid membrane casting composition that exhibits a lower imidization temperature, see G. A. Polotskaya, et al., Polymer Science, Ser. B., Volume 38, pp281, 1996 (English translation). The composition is formed by combining a polyamic acid dissolved in dimethylacetamide (DMAc) with 0.5-2.0 equivalent amount of benzimidazole. The composition is used to form composite membranes by coating a poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) porous substrate saturated with high aliphatic hydrocarbons. The coating layer is converted to polyimide by heat treatment at 150xc2x0 C. The disclosed method, however, suffered from the use of a harsh, high boiling solvent. The solvent used is difficult to remove and can destroy or otherwise adversely effect the porous substrate. As the result, the composite membrane formed has inferior gas permeation properties. Furthermore, the disclosed method can be applied only to a limited number of solvent resistant substrates.
M. Oba, et al. have reported in Journal of Polymer Science, Part A: Polymer Chemistry, Volume 34, pp 651, 1996; and in U.S. Pat. Nos. 5,753,407 and 5,756,650 that the imidization temperature of the polyamic acids can be lowered to about 150xc2x0 C. in the presence of large amount of catalysts (up to 2 equivalent per repeat unit of polyamic acid), such as p-hydroxybenzoic acid. The authors have not disclosed or implied that catalysts can be advantageously utilized to reduce imidization temperature of polyamic acid salts in membrane preparation. It is known in the art that polyimide polymers can be prepared from polyamic acid salt precursors, which are formed by neutralization of the free carboxylic acid group with a tertiary amine base. U.S. Pat. Nos. 4,290,929 and 5,719,253 disclose the use of polyamic acid solutions of tertiary amine. The following publications also disclose the synthesis of polyamic acid salts: R. J. W. Reynolds and J. D. Seddon, Journal of Polymer Science, Part C, Volume 23, pp45, 1968; and J. A. Kreuz, A. L. Endrey, F. P. Gay, and C. E. Sroog, Journal of Polymer Science, Part A-1. Volume 4, pp 2607, 1966; Y. Echigo, N. Miki, and I. Tomioka, Journal of Polymer Science, Polymer Chemistry, Volume 35, pp2493, 1997.
It has been taught in the art that amphiphilic polyamic alkylamine salts can form Langmuir-Blodgett (LB) films on water surfaces that subsequently can be converted into polyimide films, see, for example, U.S. Pat. No. 4,939,214 as well as Y. Nishikata, et al., Polymer Journal, Volume 20, pp269, 1988, and Y. Nishikata, et al., Thin Solid Films, Volume 160, pp15, 1988. Marek et al. disclosed the preparation of thin LB films for gas separation applications from dimethyldodecyl-ammonium and dimethylhexadecyl-ammonium polyamic acid salts, see M. Marek et al., Polymer, Volume 37, pp2577, 1996. The authors concluded that LB films with gas separation characteristics cannot be obtained from the short-chain tertiary amine salts of polyamic acid. Marek et al. found that to form LB films that exhibit gas separation property, one of the alkyl chains in the tertiary amine salt has to be longer than 16 carbon atoms to form an acceptable LB film.
Therefore, there still remains a need for improved methods to prepare polyimide membranes, in particular, methods that employ mild organic solvents and/or mild heat or chemical treatments in polyimide membrane preparation and result in improved permeation/separation characteristics.
The instant invention discloses improved and industrially feasible methods for the fabrication of polyimide articles, such as films, coatings and, most preferably, gas separation membranes. Polyimide articles such as membranes of the present invention are produced by a two-step process: (a) an article, such as a membrane is formed from a polyamic acid salt membrane precursor that contains the following units in its structure: 
wherein R is a substituted or unsubstituted aromatic, alicyclic, heterocyclic, or aliphatic radical. X is an ammonium ion, a phosphonium ion, a sulfonium ion, a protonated tertiary amine or a quaternary amine or a mixture thereof. The quaternary amine ion can be a heterocyclic, alicyclic or an aromatic amine ion or an ion of the following general formula: R1R2R3R4N+. The protonated tertiary amine can be a heterocyclic, alicyclic or an aromatic amine or an amine of the following general formula: R1R2R3NH. R1, R2, R3 and R4 can be the same or different and are aryl or alkyl radials, and (b) the article formed from the polyamic acid salt precursor is converted into the polyimide article, such as a membrane by thermal or chemical treatment.
In one embodiment of this invention, the solubility of PAAS polymers in mild, low boiling temperature solvents can be enhanced by addition of amines and/or water to the casting solution from which the PAAS article is formed.
In another embodiment of this invention, the thermal imidization temperature for PAAS polymer precursory article can be lowered by incorporating a catalyst into the article forming solution.
In a further embodiment of this invention, PAAS polymer precursory membranes can be chemically converted into final polyimide membranes by using a dilute solution of a dehydration agent in an inert solvent. The chemical imidization solvent system does not adversely effect the porous membrane structure. These characteristics are extremely useful for the fabrication of gas separation membranes, in particular, composite membranes with improved permeation/separation characteristics.
other features and advantages of the present invention will become apparent from the following description of the invention.